1. Field of the Invention
The present invention relates generally to radiation shielding materials for nuclear criticality control. In particular, the invention relates to composite neutron absorbing materials and methods of coating these materials on storage containers for use in spent nuclear fuel applications requiring long term storage and corrosion resistance.
2. Background Technology
The reliance on nuclear power as a method for power generation has been increasing rapidly in recent years, due to a corresponding increase in the demand for electric power throughout the world. Accordingly, the amount of spent nuclear fuel (SNF) has increased along with the need for safe methods for long term storage and disposal of these radioactive waste materials. Ideal containers for storage and transport of radioactive wastes should have the capability of safe containment for as many years as possible. There are, however, significant safety issues involved in the safe, long-term storage of SNF elements due to high levels of uranium enrichment.
In response to these issues, various thermal neutron absorbing materials have been developed for placement in close proximity to SNF elements to capture the neutrons that are emitted from the fuel to prevent nuclear criticality accidents. In conventional storage approaches, arrays of neutron absorbing materials are placed or incorporated into storage containers holding SNF elements. The typical containment system includes a shielded container which has at least one internal shell that is coated with a material capable of preventing thermal neutrons emitted from the SNF from initiating an unwanted nuclear chain reaction. Stainless steel has been frequently used as a structural component of SNF storage containers because it has good corrosion resistance and acceptable mechanical properties.
Various other approaches have been developed for containment of spent nuclear fuel. For example, U.S. Pat. Nos. 5,786,611 and 6,166,390 to Quapp et al. disclose radiation shielding containers for storing radioactive materials. The containers are formed from a concrete product including a stable uranium aggregate and a neutron absorbing material. Possible neutron absorbing materials described are B2O3, HfO2, and Gd2O3. The concrete product is formed by a liquid phase sintering process that allows the addition of the neutron absorbing additives at the same time as the formation of the uranium aggregate.
In U.S. Pat. No. 6,125,912 to Branagan et al., neutron absorbing materials are disclosed that utilize rare earth elements, such as gadolinium, europium, and samarium. These materials are formed as metallic glasses or nanocrystalline materials which can be incorporated into SNF storage containers. The method for making these materials comprises: starting with a base alloy composition which contains the rare earth element along with one or more transition metals, forming a melt of the base alloy, and then rapidly solidifying the base alloy melt using surface quenching or atomization techniques. The resulting neutron absorbing material is in the form of an amorphous glass or a material with partial crystallinity and partial amorphicity, or in the form of a powder.
Other typical neutron absorbing materials that have been used in SNF storage containers include the following: boron carbide in an aluminum matrix; boron carbide in an elastomeric matrix; boron carbide in a resin matrix; aluminum-boron alloys; borated stainless steel alloys; and stainless steel clad neutron absorbing materials. Many of these materials, however, have been shown to have the following disadvantages: aluminum-based materials have inferior corrosion resistance in some wet storage environments; the elastomeric and resin-based materials are susceptible to radiation damage which causes embrittlement, and the borated stainless steels have weldability and mechanical property (low ductility/fracture toughness) problems.
With respect to borated stainless steels, these have typically been used since boron has a large absorption cross section for thermal neutrons. However, borated stainless steels have a limited range of usefulness because of certain metallurgical properties, for example, an inability to be easily welded into the required structural shapes for the containers. In addition, boron is somewhat soluble in water, which can result in eventual deterioration of a container made from a borated stainless steel. Further, the bombardment of borated stainless steel by the neutrons emitted by radioactive material has the effect of reducing its effectiveness as a neutron absorber, making it an unsuitable material for long term safe containment of radioactive waste products.
Accordingly, it would be desirable and advantageous to provide improved aterials and methods for making containers more safe in the transport and storage of radioactive waste.